The Pentagon has deployed about 100 troops — including more than 50 Marines attached to a Fleet Antiterrorism Security Team to the U.S. Embassy compound in Baghdad, Iraq, to help protect diplomatic personnel and property.

Meanwhile, President Obama is considering miltiary action against the Islamic insurgents, who have seized vast swaths of northern Iraq and are moving south toward the capital. Several U.S. warships have moved into the Persian Gulf, where they provide “the commander in chief additional options to protect American citizens and interests in Iraq, should he choose to use them,” said Rear Adm. John Kirby, a Pentagon spokesman, on Monday.

The arrival of FAST Marines and a contingent of U.S. soldiers on the ground in Iraq on Sunday marked the first operational deployment of U.S. troops there since the withdrawal of combat forces in December 2011. Pentagon officials declined to identify the Army unit deployed to Baghdad. The Marine platoon is based out of nearby Bahrain, and is tasked with protecting American personnel and property, said Master Sgt. William Price, a spokesman for Marine Corps Forces Central Command.

FAST Marines are the traditional go-to assets when U.S. embassies require reinforcement in times of crises. The Marine Corps has two more forward-deployed FAST elements in Spain and Japan.

“This is a temporary thing,” Kirby said Monday. “There is no intention that this is any kind of permanent plus up. They are there temporarily, to assist with some relocation of some personnel who work at the embassy. They are not engaged in ferrying to and fro anyone. No military aircraft … is being used to ferry these folks.”

On Monday, Defense Secretary Chuck Hagel ordered the amphibious transport dock Mesa Verde, part of the Bataan Amphibious Ready Group, to enter the Persian Gulf. It joins the carrier George H. W. Bush, which Hagel ordered to enter the Gulf on Saturday.

The carrier brings F/A-18 Super Hornets that could provide air strike capability over Iraq. The Mesa Verde carries more Marines, all members of the 22nd Marine Expeditionary Unit, along with MV-22 Osprey transport aircraft that will be in standby in case the State Department needs support to completely evacuate the embassy in Baghdad.

Also entering the Persian Gulf Saturday was the guided-missile cruiser Philippine Sea and the guided-missile destroyer Truxtun. The ships carry Tomahawk missiles that could reach inland Iraq.

Insurgents with an al-Qaida-offshoot group known as the Islamic State of Iraq and Syria, or ISIS, appear to be approaching Baghdad from three sides: from Anbar province in the west, from Samara and Tikrit in the north, and also from northwestern Diyala province, which is within 50 miles of the capital. Reports suggest their advance has slowed since approaching Baghdad and moving into places with large Shiite populations.

While the ISIS force is not large — estimates suggest no more than several thousand personnel inside Iraq — the group’s stunning series of victories largely resulted from the widespread desertion of thousands of Iraqi security forces. An ISIS force of about 800 fighters reportedly routed two Iraqi Army divisions totaling about 30,000 troops in Mosul during a brief battle last week.

As the Iraqi troops have melted away, the ISIS force has seized some of its American-made military gear, including Humvees and possibly even some helicopters, according to photos released by ISIS forces. But U.S. military officials say it remains unclear how much weaponry the ISIS force took and what is operational impact might be.

For now, the U.S. Embassy in Baghdad is scheduled to remain open, but some staff will be temporarily relocated amid growing violence and instability in Iraq, according to a statement Sunday from Jen Psaki, a State Department spokeswoman.

Kirby said the relocation of those personnel is being facilitated aboard commercial, charter and State Department aircraft. But the military has airlift assets “at the ready” should the State Department request them, he added.

The Marines’ ability to quickly deploy and enhance security at diplomatic posts is one of the service’s central missions since the deadly 2012 attack on the U.S. consulate in Benghazi that left four Americans dead, including Ambassador J. Christopher Stevens.

When Congress demanded better protection of diplomatic facilities and personnel across the globe, the Marine Corps added 1,000 new embassy security guards. That included the creation of a new group called the Marine Security Augmentation Unit made up of teams of trained embassy guards. Those teams can be summoned directly by an ambassador if intelligence indicates the threat of an attack.

The Marine Corps also stood up a new crisis response force, made up of nearly 1,000 Marines. That unit is based in Europe, and is designed to respond to diplomatic crises in Africa. Plans call for a creation of similar crisis-response force in the Middle East.

The U.S. military withdrew its forces in 2011 in large part because the Iraqi government refused to grant U.S. troops legal immunity from prosecution in local Iraqi courts. Since 2011, the number of U.S. troops in Iraq has been capped at a few hundred uniformed personnel working at the embassy, and those troops have legal protections as embassy workers despite the lack of a formal Status of Forces Agreement between the U.S. and Iraq.

Kirby said Monday the additional troops sent into Iraq this weekend are not at risk legally. “I would expect that all the legal protections they need, they have,” he said.